This invention relates to a dielectric printing paper, of the type used to selectively attract toner particles by use of differential electrostatic potential on the surface of the paper, and to an improved versatile process for making such a paper.
Electrostatic printing paper, or dielectric papers suitable for electrostatic printing as they are more properly described, are well known to the art. These papers are to be distinguished from the photosensitive papers which are commonly used with office copying equipment.
Dielectric printing is based on forming a charged area on a dielectric surface by electron-beam, or some other such selective surface charging means. The charged area is then directly contacted with a toner selectively attracted to the areas of the paper made electrically receptive to it. There is no intermediate light-caused discharging process, and photoconductive materials are not generally useful in dielectric printing processes using liquid toners and wherein, for example, a print speed of 18,000 lines per minute is typical. In general, dielectric copy sheets are used in high-speed copying processes. Papers heretofore used in such processes tend to be expensive because of their utilization of expensive organic conductivity-imparting additives, of relatively expensive coating substrates, and of relatively expensive dielectric coating procedures.
It has long been a problem to provide electrostatic printing papers having a high-filler content. For example, U.S. Pat. No. 3,847,661 is typical of a coating laid down from a liquid medium. There are a number of problems caused by such processes. If the liquid medium is a solvent for a polymeric matrix, then there is substantial contamination of the surfaces of filler product with the polymer as evaporation of the solvent takes place. This interferes with ink-receptively of the filler reducing substantially any absorbency of said filler for eventual use in imaging. If the liquid medium is water, then there is an increased chance of excessive filler polymer segregation and, moreover, there is often a tendency to disrupt the water-laid fibers of the paper being coated. Another problem with aqueous coating of the dielectric layer is the fact that one must, from a practical point of view, limit selection of the electrolyte, used to impart a degree of conductivity to the substrate paper, to one which has relatively low water solubility. Even with such a limitation made, the use of aqueous coating procedures in manufacturing operations can result in unwanted contamination of the dielectric coating with the substrate electrolyte. The process of U.S. Pat. No. 3,847,661 is characterized by a substantially continuous polymer layer and is limited to low pigment levels. U.S. Pat. No. 3,956,562 to Shibata discloses a process for increasing the filler content of coatings by pre-coating the particular with a plastic envelope which remains on the surface of the particle in the coating and, to that extent, interferes with the imaging performance contributed by the inorganic filler present on the surface of the paper. Even with the pretreatment, however, total particle content of the coating is limited.
In the above discussion, the term liquid coating systems relates to those using volatile organic solvents or water as coating vehicles of low viscosity.
It should be noted that this discussion of the prior art is made with knowledge of the present invention and after having an opportunity to evaluate the advantages of the present invention and the probable reasons for those advantages, in light of drawbacks of the prior art processes. It is not to be inferred that the disadvantages of the prior art or the reasons for such disadvantages were realized by prior artisans before the present invention was made.